1. Field of the Invention
The present invention relates to a control system for controlling a motor arrangement for differentially driving left and right wheels of a motorized vehicle, in particular but not exclusively, a powered wheelchair.
2. State of the Art
An electrically powered wheelchair typically has two independently driven electric motors, one for driving a left wheel of the wheelchair and the other for driving a right wheel of the wheelchair. The differential speed of these motors controls the forward speed and turn rate of the wheelchair.
A typical wheelchair is provided with an input device, usually a two axis joystick, to which a user applies stimuli in order to generate forward speed and yaw rate reference signals. A control module receives the reference signals from the input device, and processes the signals to calculate left and right motor speed reference signals that are compared to signals indicative of actual measurements of the current left and right motor speeds. The differences between the reference motor speed signals and the actual motor speed signals are used by a closed loop control system to apply voltages to the motors to drive the wheels to maintain the wheelchair at the user demanded speed and heading.
Such a closed loop control is usually achieved by what is known as the IR compensation technique. IR compensation is a method used to compensate for the fall in a motor's speed due to the voltage drop across its armature resistance (R) when current (I) is flowing in the motor's armature winding. IR compensation causes the velocity control loop to increase the motor voltage and motor current to stabilise the motor's speed when the motor load increases. If the R term is set too high, the positive feedback mechanism inherent in the IR compensation method causes a control problem and the control loop becomes unstable.
For example, as load is applied the motor, the over compensated IR term in the velocity feedback calculation causes the velocity control loop to increase the motor voltage to an extent where the motor's speed and current are forced to increase. Consequently, the velocity loop reacts to further increase the motor voltage, which further increases the motor speed and current. This runaway effect of the motor velocity causes speed instability of the motor's closed velocity loop. It is therefore not possible to compensate for the full armature resistance of the motor and this leads to an error in the velocity feedback signal which can, under certain driving conditions, affect the controllability of the wheelchair.
There are several potential wheelchair control problems that may result from anomalies in IR compensation. For example, when manoeuvering at low speeds in restricted spaces, if the wheelchairs' motor loads do not remain constant, the user will need to constantly make corrections to keep the wheelchair on a desired course by varying the displacement of the joystick. This is because the error in the velocity feedback signal caused by the low R term varies with motor load and the only way to maintain a constant velocity is to compensate for this error by varying the input demand. This makes manoeuvers in constricted, cluttered spaces difficult and it often leads to collisions because the user lacks sufficient fine, low speed control. Examples include passing through narrow doorways, driving along narrow corridors and docking at a table. These manoeuvers are most difficult when driving on high friction surfaces such as carpets.
Control problems may also occur when driving at high forward speeds, because any sudden asymmetric loading of the right and left wheelchair motors will tend to cause the motor with the decreasing load to speed up and the motor with the increasing load to slow down which results in the wheelchair veering off its intended course.
Furthermore, when driving a wheelchair across a camber, asymmetric loading of the right and left wheelchair motors causes the wheelchair to veer down the camber even though the user demands to drive across it.
Also when driving over obstacles, asymmetric loading of the wheelchair motors can cause the wheelchair to deviate sharply from its intended direction of travel.
U.S. Pat. No. 6,202,773 describes a motorized wheelchair provided with a rate-of-turn feedback sensor and with forward/reverse motion, lateral motion, and vertical motion acceleration feedback sensors that are integrated into a closed-loop wheelchair servo control system to differentially control the rotational speed of a pair of motor driven wheels.